/Engine/Plugins/FX/Niagara/Source/Niagara/Public/NiagaraCommon.h
TODO: Custom will eventually mean that the default value or binding will be overridden by a subgraph default, i.e. expose it to a "Initialize variable" node. TODO: Should we add an "Uninitialized" entry, or is that too much friction?
enum ENiagaraDefaultMode { Value = 0, Binding, Custom, FailIfPreviouslyNotSet, }